Gene expression in two hybrid cell types constructed by nuclear transplantation will be examined. In the first type, mouse fibroblast (A9) nuclei were transplanted into rat hepatoma (HTC) cytoplasts. Analysis indicated that a liver-specific steroid-inducible tyrosine aminotransferase (TAT), apparently mouse in origin, was activated in the hybrid cells. Experiments designed to acquire definitive biochemical evidence that silent mouse genes were indeed activated will be performed. These will include peptide mapping of the induced TAT and a two-dimensional gel electrophoresis analysis of polypeptide synthesis in the hybrids and two parent cell lines. The mechanism of activation will be investigated by experiments in which A9 nuclei will be transplanted into HTC cytoplasts in various metabolic states. Attempts will be made to devise an assay for the hypothetical activating substance. In the second type of hybrid cell, dormant erythrocyte nuclei were transplanted into mouse or chick fibroblast cytoplasts. Synthesis of two forms of alpha globin chain, but no beta chain - the third major form made in mature erythrocytes - was detected in the hybrids. Experiments designed to determine whether the block in beta globin synthesis is at the transcriptional or translational level will be performed. They will include an assessment of globin chain synthesis in fibroblast cytoplasts into which globin message is introduced in other ways, and direct analysis of the RNA species made in hybrids by DNA-RNA hybridization techniques. Both types of hybrid cell systems discussed here may prove useful in investigations into the relationship of chromatin structure to gene expression.